So, it actually newfs'd the mounted hammer fs, then you can unmount
the hammer and mount the device as UFS.

I think it works because newfs looks into the mounted devices, but
there is no /dev/xyz, but only the label... so it goes on happily and
newfs maybe does not check for existing superblocks (like newfs.xfs)
and if it checks maybe it doesn't know how a hammer one looks like.